1. Field of the Invention
The present invention relates to a color image forming apparatus and method. More particularly, it relates to a color image forming apparatus and method which handles image signals associated with different densities of each of the same color families.
2. Description of Related Art
A color ink jet recording apparatus has been conventionally known as one of the color image recording apparatuses. Such a type of color ink jet recording apparatuses has a problem in that dots are often noticeable in highlighted portions. This problem has been dealt with by trying to improve the graininess of highlighted portions by using a plurality of coloring materials of different densities of the same color families such as thin black ink, thick black ink, thin cyan ink, thick cyan ink, thin magenta ink, thick magenta ink, thin yellow ink, and thick yellow ink.
FIG. 1 is a perspective view schematically illustrating a color ink jet recording apparatus as an example of such types of color ink jet recording apparatuses.
In FIG. 1, a roll of recorded medium 5 is transported with transporting rollers 1 and 2, held between a feed rollers 3, and is transported in the direction f by a subscanning motor 4 joined to the feed roller 3. Guide rails 6 and 7 are disposed in parallel across the recorded medium 5, so that recording heads 9 mounted on a carriage 8 scan across the medium.
The carriage 8 is loaded with eight recording heads each for thin black, thick black, thin cyan, thick cyan, thin magenta, thick magenta, thin yellow, and thick yellow, which are designated by 9LBk, 9DBk, 9LC, 9DC, 9LM, 9DM, 9LY and 9DY, respectively. Eight ink reservoirs, which are not shown in this figure, are disposed such that each of them corresponds to one of the eight recording heads.
The recorded medium 5 is incremented intermittently by an amount equal to the recording width of the recording heads 9, which record an image by discharging ink droplets in response to image signals by scanning in the P direction while the recorded medium 5 is at halt.
A signal processing system of such a color image recording apparatus is generally arranged as shown in FIG. 2. Since the arrangement is the same for each one of the four systems of black, cyan, magenta and yellow, only one system is illustrated here.
In FIG. 2, a thick/thin sorting portion 31 separates an input image signal into an image signal associated with thin ink and an image signal associated with thick ink using a sorting table having characteristics as shown in FIG. 3, and supplies the image signals to a binarizing portion 32.
The binarizing portion 32 binarizes the respective image signals, and supplies them to head drivers 33L and 33D, which drive recording heads 9L and 9D. Thus, an image is recorded.
Details of FIG. 3 will be described below. If the input image signal (8 bits) takes one of the values 0-127 before the thick/thin sorting, only the image signal associated with the thin ink recording head 9L is outputted. The purpose of this is to represent optical density by recording many dots associated with thin ink, thereby preventing the individual dots from appearing, and improving the image quality.
On the other hand, when the input image signal takes one of the values 128-255 expressed in the 8-bit notation, the value of the image signal associated with the thin recording head 9L after the thick/thin sorting is reduced as the value of the input signal before the thick/thin sorting increases.
This is performed for the following reason. The Value of the image signal associated with the thick ink recording head 9D is increased in the range of 128-255 of the input signal value before sorting, so that the recording volume of the thick ink is increased to represent the optical density. In this case, unless the volume of the thin ink is decreased, the total ink volume will exceed an allowable ink volume of a recorded medium when a plurality of inks are deposited, thereby resulting in an overflow of the ink.
An image recording apparatus that achieves gradation by using a plurality of inks of different densities is-disclosed in Japanese Patent Publication No. 14905/1990 (U.S. Pat. No. 4,635,078) assigned to the present assignee. This apparatus makes it possible to express gradation that could not be expressed by a single type of dots.
In such a color image recording apparatus, however, it is difficult to obtain N color materials of different densities of the same color family (such as thin cyan ink and thick cyan ink) in such a manner that their hues perfectly coincide with each other.
This presents a problem in that high color reproducibility of output images cannot be achieved when the color image recording apparatus, which performs a color masking process on the basis of the input image signal before the thick/thin sorting, is applied to a copying machine or the like.
Furthermore, since the conventional color ink jet recording apparatus obtains an image using a pair of thick and thin ink heads, a complete image cannot be obtained if one of the thick and thin ink heads is damaged.